Laser interference lithography (LIL) is an inexpensive approach for fabricating periodic structures, wherein LIL utilizes two mutually-interfered coherent laser beams to form periodic interference fringes. While the photosensitive material (e.g. photoresist) is exposed to the interference fringes, the patterns of the periodic interference fringes can be transferred to the photoresist to form periodic grating structures, and furthermore, the periodic grating structures on the photoresist can be transferred to various materials by etching techniques. Low-cost and high-throughput LIL is widely used for distributed feedback semiconductor laser fabrication for telecommunication applications, which requires a grating periodicity of around 200 nm uniformly over a 2-inch III-V epi-wafer. In addition, LIL can be applied to fabricate patterned sapphire substrates for light-emitting diode applications, and fabricate wire-grid polarizer for display, wherein the formation of short-period gratings uniformly over a larger sample area is the major objective of LIL.
A Lloyd's mirror interferometer is the most common approach to conduct LIL process, in which a reflecting mirror is placed perpendicular to a sample holder. A laser beam with an enlarged light field is utilized to illuminate the reflecting mirror and the sample holder, simultaneously, wherein the beam reflected from the reflecting mirror illuminates the sample holder as the second beam for two-beam interference process. The LIL system equipped with a Lloyd's mirror is simple in system setup and not susceptible to environmental disturbance, but it can not be applied to generate uniform periodic structures over a large sample area because the energy distribution of the two beams in the system are non-uniform and respectively right and left sides of Gaussian intensity profile. While the two beams interfere mutually, the exposure dose on the sample region close to the reflecting mirror is higher than that far away from the reflecting mirror, so the uniformity of the grating structure on the sample is not sufficient.
A novel tunable two-mirror LIL system was disclosed in U.S. Pat. No. 8,681,315 (Mao et al, 2011), wherein two additional reflecting mirrors are utilized in this novel system to adjust the paths of two expanded beams for different interference fringe periodicity, without altering the configuration of other optical components, and the grating structure over 4-inch can be obtained. However, the uniformity of the grating structure is still insufficient and the issue arise from Gaussian intensity profile still exists. For this reason, a laser interference lithography system with flat-top intensity profile to solve the existing problem in prior art is required.